The present invention relates to electronics, and, more particularly, to a voltage regulator. The voltage regulator includes a capacitor providing a regulated voltage, a regulation switch for connecting the capacitor to a voltage source, and a regulation circuit for closing the regulation switch when the regulated voltage is below a first reference voltage.
Voltage regulators have various applications in the field of electronics. For example, voltage regulators deliver regulated voltages to the external pins of a microprocessor. FIG. 1 shows a conventional voltage regulator 1 incorporated into a microprocessor 2. The microprocessor 2 is supplied with a battery 3 having an internal resistance ri and provides a voltage VDD.
The regulator 1 comprises an external capacitor CREG providing, on its anode, a regulated voltage VREG. The anode of the capacitor CREG is coupled to the anode of the battery 3 by a regulation switch SW1 presenting a serial resistance R1 equal to zero or a small value. The switch SW1 is driven by the output of a follower circuit CP which receives the regulated voltage VREG on its positive input and a reference voltage VREF on its negative input.
The voltage VREF is conventionally a band-gap voltage, and presents good stability versus temperature, and is generated by PN junction diodes and current mirrors. The follower circuit CP is controlled by a signal PDW (Power-Down) and the anode of the capacitor CREG is coupled to ground by a switch SWOFF driven by the signal PDW. When the signal PDW is equal to 1, the follower circuit CP is OFF and the switch SWOFF is closed. The regulator 1 is stopped and the capacitor CREG is discharged.
The drawback of such a regulator is that it presents a high current consumption at start-up. When the signal PDW is set to 0, the capacitor CREG is discharged and the voltage VREG is equal to zero. The output of the follower circuit CP passes to 0 and the switch SW1 closes. The application of the voltage VDD to the capacitor CREG causes a high current drain and a significant drop of the voltage supply VDD of the microprocessor 2 because of the internal resistance ri of battery 3. If, at the same moment, other elements of the microprocessor 2 consume non-negligible current levels, it may occur that the voltage VDD becomes less than the minimal working voltage of the microprocessor 2 so that microprocessor 2 will not operate.
In view of the foregoing background, an object of the present invention is to limit the starting current of a voltage regulator without increasing the serial resistance of the regulation switch.
This and other objects, advantages and features are provided by a voltage regulator of the above described type comprising at least one ballast switch arranged in parallel with the regulation switch, and a circuit for opening the regulation switch and closing the ballast switch at least during a starting phase of the voltage regulator until the capacitor is at least partially charged.
According to one embodiment, the voltage regulator comprises a circuit for opening the regulation switch and for closing the ballast switch when the regulated voltage is below a second reference voltage lower than the first reference voltage. The second reference voltage is preferably a fraction of the first reference voltage.
A common control signal for the regulation switch and the ballast switch is preferably applied to the regulation switch by an inhibiting switch. The inhibiting switch is driven by a signal delivered by a comparator receiving as inputs the regulated voltage and a second reference voltage lower than the first reference voltage.
According to another embodiment, the voltage regulator comprises a follower circuit receiving as inputs the regulated voltage and the first reference voltage. The follower circuit delivers a regulation signal, and a comparator receives as inputs the regulated voltage and a second reference voltage lower than the first reference voltage. The regulation signal is applied to the control input of the ballast switch and to the control input of the regulation switch by an inhibiting switch. The output of the comparator is applied to the control input of the inhibiting switch.
The ballast switch is preferably a MOS transistor comprising a non-negligible intrinsic resistance. The voltage source is preferably an electric battery.
Another feature of the present invention relates to an integrated circuit comprising a voltage regulator as disclosed herein.